This invention relates to an improved electronic ballast circuit used for gaseous discharge lamps operating on direct current (DC) voltage. More particularly, it relates to apparatus for permitting reversal of the DC voltage applied to a lamp without impressing excessive voltage stresses on switching components.
Gaseous discharge lamps, such as, for example, fluorescent lamps, are basically designed to operate from alternating current (AC). However, when operated with AC current, the lamp exhibits a phenomenon known as AC flicker effect. In essence, the lamp illumination varies as the applied AC current switches from a positive to a negative half-cycle. Although this flicker effect is not apparent to the human eye, it is often sensed by the faster responding photo-recepters used in photocopiers. As machines tend toward faster speed, flicker effect results in more distortion of the copy fidelity.
In order to avoid the flicker effect, most photocopiers utilize a highly regulated DC current for the fluorescent lamp in order to reduce all components of light intensity versus time to a negligible level. However, when a fluorescent lamp is operated at a constant DC current, the fluorescent lamp goes through a process of mercury migration. This phenomenon, i.e., mercury migration, results in a nonuniform brightness of the lamp from one end to the other. The mercury migration process generally starts slowly but eventually ends in a quite noticeable difference in lamp intensity across the fluorescent lamp. Such a variation in intensity across the lamp produces a photocopy which varies from dark to light across its face.
An additional problem encountered with the operation of fluorescent lamps is an effect known as anode darkening. Anode darkening is caused by an overheating of the anode of the lamp due to constant excessive bombardment of electrons. Overheating causes damage to the phosphors at the anode end of the lamp and results in no light being emitted near the anode end after a few hours of operation on DC current. As with the previous intensity problem with fluorescent lamps, this latter problem also produces nonuniform reproduction in the photocopying process.
The undesirable effects of mercury migration and anode overheating can be greatly reduced if the lamp current polarity is periodically reversed and if the illumination time is limited to the minimum time actually required to perform the photocopying function. Polarity reversal thus will result in a much more uniform brightness with better optical quality and greatly increased useful lamp life.
As is well known, a fluorescent lamp requires a high voltage initial application in order to ionize the gases within the lamp and start the current conduction through the gas. Once ionization has occurred, i.e., the lamp is lit, current can be maintained by a relatively low voltage. A lamp may require, for example, a starting voltage in the order of magnitude of 1,000 volts, whereas the maintaining voltage may be only in the order of magnitude of 80 volts. The high voltage start pulse may be applied by paralleling it with the lamp or by applying the pulse in series with the lamp. In either method, the high voltage appears across the lamp and provides the voltage necessary for the lamp to begin conduction. In those prior art systems which provide for polarity reversal across the lamp, there are also provided high voltage relays arranged about the lamp to provide for its effective reversal with respect to the DC voltage applied to maintain current conduction through the lamp. In all of these prior art systems, the high voltage start pulse, in addition to being applied across the lamp was also applied across the high voltage relays. The use of high voltage relays was necessary to prevent relay contact arcing rather than starting the fluorescent lamp. The use of high voltage relays in such circuits is generally undesirable because of their relatively high cost and tendency to weld contacts when operated on DC.